LDMOS transistor with self-aligned source/backgate and photo-aligned gate

ABSTRACT

An embodiment of the present invention is a method of fabricating power and non-power devices on a semiconductor substrate, the method comprising: forming alignment marks in the substrate (100); introducing a dopant of a first conductivity type into the substrate to form high-voltage tank regions (103); annealing the dopants (105); introducing dopants of the first conductivity type and a second conductivity type in a region in the high-voltage tank region (109); annealing the dopants of the first and the second conductivity type to form a second region within a third region, both within the high-voltage tank region, due to the different rates of diffusion of the dopants (110); and forming gate structures after the annealing of the dopants of the first and second conductivity types (122).

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of application Ser. No. 08/071,156, filed Jun. 2, 1993, now U.S. Pat. No. 5,248,895 which is a continuation of Ser. No. 07/857,360, filed Mar. 25, 1992 now U.S. Pat. No. 5,242,841.

The following coassigned patent applications are hereby incorporated herein by reference:

    ______________________________________                                         Ser. No.    Filing Date TI Case No.                                            ______________________________________                                         857,533     March 25, 1992                                                                             TI-16227                                               ______________________________________                                    

1. Technical Field of the Invention

The present invention is in the field of semiconductor processing.

2. Background of the Invention

In many applications it is desirable to have a variety of logic devices, memory devices, and devices capable of withstanding large transients (hereon referred to as power devices) all on a single substrate. One such type of power device is a double-diffused metal-oxide-semiconductor transistor (DMOS).

Heretofore most lateral double-diffused "metal" oxide semiconductor, LDMOS, structures built on a substrate with one or more other device structures were formed by first forming a high-voltage tank. After formation of the high-voltage tank, a second low-voltage tank (typically used as the backgate), of opposite conductivity type to that of the first tank, was formed within the first high-voltage tank, and active regions and gates were aligned to these tanks. The alignment of these to other existing structures is difficult.

Purely self-aligned LDMOS transistors cannot be easily integrated because a level of polysilicon is required early in the process. A purely photo-aligned device is difficult to control because the backgate leaves no marks for alignment, and channel length is alignment dependent.

It is, therefore, an object of the invention to form source/channel regions that are self-aligned and formed prior to the formation of the gate. It is a further object of the invention to perform a majority of the high-temperature process early in the process flow. Further objects and advantages of the invention will become apparent to those of ordinary skill in the art having reference to the following specification together with the drawings.

SUMMARY OF THE INVENTION

One aspect of the present invention is a process flow which can be used to fabricate power and non-power devices on a single substrate while maintaining superior performance. More specifically, the process flow of the present invention was created to enhance the performance of the power devices, and at the same time reduce the number of process steps by incorporating the process steps used to enhance the performance of the power devices to fabricate the non-power devices, without degrading their performance. Another aspect of the present invention is a process used to fabricate power devices and non-power devices in which the channel length of the devices is defined by the different diffusion rates of the dopants used to form the channel and surrounding tank structures.

An embodiment of the present invention is a method of forming a device on a semiconductor substrate. The method comprises the steps off selectively forming a first region of a first conductivity type, preferably n-type, in the semiconductor substrate; subjecting the substrate to a first substantially elevated temperature, preferably after forming said first region; introducing dopants, preferably by implantation, of the first conductivity type and a second conductivity type, preferably p-type, into a region in the first region, the dopants having different diffusion rates; subjecting the substrate to a second substantially elevated temperature thereby forming a second region within a third region, both within the first region, due to the different diffusion rams of the dopants; forming field oxide; forming a gate structure after the selectively forming the second region; and forming source/drain regions aligned to the field oxide, the gate structure, or both.

Another embodiment of the present invention is a method of fabricating power and non-power devices on a semiconductor substrate, the method comprising: forming alignment marks in the substrate; introducing a dopant of a first conductivity type into the substrate to form high-voltage tank regions; annealing the dopants; introducing dopants of the first conductivity type and a second conductivity type in a region in the high-voltage tank region; annealing the dopants of the first and the second conductivity type to form a second region within a third region, both within the high-voltage tank region, due to the different rates of diffusion of the dopants; and forming gate structures after the annealing of the dopants of the first and second conductivity types.

A third embodiment of the present invention is a method of making a LDMOS transistor, comprising the steps off forming a DWELL region, preferably by first introducing dopants of opposite conductivity types into the high voltage tank region using the single mask and then annealing the dopants of opposite conductivity type thereby forming a first doped well within a second oppositely doped well, in a high voltage tank region of a LDMOS transistor by using a single mask; and forming a gate for the LDMOS transistor after forming the DWELL region.

A fourth embodiment is a method of fabricating power and non-power devices on a semiconductor substrate of a first conductivity type, the method comprising: introducing dopant of a second conductivity type opposite the first conductivity type into the substrate to form high-voltage tank regions; annealing the dopants used to form high-voltage tank regions; introducing dopant of a second conductivity type into the substrate to form low-voltage tank regions; introducing dopant of a first conductivity type into the substrate to form high-voltage tank regions; introducing dopant of a first conductivity type into the substrate to form low-voltage tank regions; introducing dopants of the first conductivity type and a second conductivity type in a region in the high-voltage tank region of the second conductivity type to form a dwell; annealing the dopants of the first and the second conductivity type used to form high-voltage tank regions, low-voltage tank regions, and the dwell, and wherein the first and second conductivity type dopants used to form the dwell form a first dwell region within a second dwell region, both within the high-voltage tank region formed of a second conductivity type, due to the different rates of diffusion of the dopants; forming thick gate insulating layer on portions of the substrate overlying the high-voltage tank regions of the first and second conductivity types and the first and second dwell regions: forming thin gate insulating layer on portions of the substrate overlying the low-voltage tank regions of the first and second conductivity types; forming gate structures on the thick and thin gate insulating layers; and forming source/drain regions in the substrate aligned to at least one edge of the gate structures.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described in conjunction with the accompanying drawings, in which:

FIG. 1 is a high level semiconductor process flow diagram;

FIGS. 2a-2k are highly magnified sectional views of different portions of an integrated circuit manufactured in accordance with the process flow of FIG. 1, showing successive stages in the fabrication thereof;

FIG. 3a is a sectional view of a lateral DMOS transistor manufactured in accordance with the process flow of FIG. 1;

FIG. 3b is a plan view of the lateral DMOS transistor shown in FIG. 3a, FIG. 3a being taken substantially along line 3a--3a of FIG. 3b;

FIG. 4a is a more detailed schematic sectional view of a vertical DMOS transistor manufactured in accordance with the process flow of FIG. 1;

FIG. 4b is a plan view of the vertical DMOS transistor shown in FIG. 4a, FIG. 4a being taken substantially along line 4a--4a of FIG. 4b;

FIG. 5a is a detailed schematic sectional view of a vertical NPN transistor manufactured in accordance with the process flow of FIG. 1;

FIG. 5b is a plan view of the lateral NPN transistor shown in FIG. 5a, FIG. 5a being taken substantially along line 5a--5a of FIG. 5b;

FIG. 6a is a detailed schematic view of a vertical NPN transistor with a buried layer manufactured in accordance with the process flow of FIG. 1;

FIG. 6b is a plan view of the vertical NPN transistor shown in FIG. 6a, FIG. 6a being taken substantially along line 6a--6a of FIG. 6b;

FIG. 7a is a detailed schematic sectional view of a lateral diffused MOS (LDMOS) n-channel transistor manufactured in accordance with the process flow of FIG. 1;

FIG. 7b is a plan view of the LDMOS transistor shown in FIG. 7a, FIG. 7a being taken substantially along line 7a--7a of FIG. 7b.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Integrated Process

FIG. 1 is a process flowchart that gives an overview of a novel process used in fabricating an integrated circuit. Before discussing specific steps of the process, some of its advantages are described.

Among the advantages provided, the process shown in FIG. 1 is modular: where particular devices are not required, some of the process steps shownin FIG. 1 may be omitted. For example, if an integrated circuit does not require EEPROM cells, then a tunnel diode step 120 would be left out. If the particular chip to be fabricated does not require power transistors having buried drain regions or collectors, then a (n+) buried layer step 101, an EPI step 102, and a deep (n+) step 104 may be omitted. If the device does not require FAMOS EPROM cells, a FAMOS floating gate step 115 and a EPROM stack step 124 may be omitted. Additions to the process can bemade as well for modifications of illustrated cells, as will be explained hereafter.

In addition to the modular nature of the process shown in FIG. 1, it also includes process steps at the beginning of the process flow which are typically performed at the end of the process flow. An advantage of this is that key parameters of the invention, such as channel length, are defined by the diffusion rate of the dopants and not by the placement of the tank with respect to a specific structure.

The inventive process provides a unified set of process parameters that canbe applied to distinctly different devices. This allows design rules for the different devices to be stored in a library. An integrated circuit designer may select different devices from this library with the assurancethat a process is available for building them, and that the devices so selected are compatible with this process. This considerably reduces design time for chips having new designs.

The semiconductor process flow of FIG. 1 additionally allows both power devices, devices typically supplied with unregulated power supplies and capable of withstanding transients as high as 60 volts, and non-power devices, devices typically subjected to lower transients, to be fabricatedon the same semiconductor chip with a minimum number of process steps and aminimum number of masks. The process flow is arranged to provide minimum amounts of heat cycling to each of the completed devices on the chip. By placing high temperature steps generally at the beginning of the process, later formed device structures are less likely to be damaged.

Turning now to a description of the process, it is to be noted that while the inventive process of FIG. 1 is described in conjunction with the fabrication of devices in a (p) type silicon substrate and epitaxial layers, the process has application to (n) type semiconductor materials and to other semiconductors. Since the process provides a unified set of process parameters that can be applied to distinctly different devices, the overall process is first discussed generically without reference to making a specific device. Accordingly, no cross-sectional drawings are provided for this discussion. Table 1, described later, and the cross-sectional FIGS. 2-6 relate the process to specific device types.

In FIG. 1, alignment marks are formed by etching targets into the a p-type semiconductor substrate in ALIGN step 100. The alignment marks define regions within the semiconductor substrate where device types may be formed, and are preferably used to align most of the masks used to form high-voltage and low-voltage tanks. For example, the alignment marks may designate low power CMOS regions and high power LDMOS regions. Next, process step 101 forms a (p-) epitaxial silicon layer and selectively creates an (n+) buried layer in the p-type epitaxial silicon layer. The (n+) buried layer may be used as the drain for vertical double diffused "metal " oxide semiconductor (VDMOS) n-channel power transistors. A buried (n+) layer is also used as the collector for a vertical npn bipolartransistor, as will be later described. A laterally separate (n+) buffed layer may be created for each VDMOS transistor, or one such layer may be used for several such transistors if these are intended to be connected inparallel. The buried (n+) layer, combined with deep (n+) diffusions, can form isolated NMOS transistors and (p-) resistors.

The (n+) buried layer is covered in step 102 by a layer of epitaxial (p-) silicon of a thickness suitable to allow subsequent thermal processing. The (n+) buried layer is thus positioned between two (p-) epitaxial layers. After creating the (n+) buried layer, step 103 fabricates (n-) transistor tanks for the high voltage power transistors. These tanks are large diffused regions in which the power transistors are fabricated. One technical advantage of the invention is that the some implantation used tocreate the high-voltage tanks for the power transistors are used to create tanks for other devices, such as n-channel 18-volt EEPROM gating transistors. Forming the power transistors in tanks isolates the sources from the substrate, allowing high-side driver circuit configurations. A high side driver configuration consists of the LDMOS transistor dram coupled to circuitry or a power supply and the source coupled to an outputload.

At step 104, a deep (n+) implant occurs in order to connect the (n+) buriedlayers to surface contacts for the vertical transistors. The (n-) high voltage diffusion and deep (n+) implants are annealed at high temperature in TANK DRIVE step 105.

Steps 101, 102, and 104 may be skipped if associated structures are not required for a given IC design.

At step 106, low-voltage device (n-) tanks are created to enclose conventional low-voltage (V_(dd) ≦volts) logic field effect transistors, and components of, for example, vertical and lateral DMOS n-channel power transistors, and drain-extended n-channel power transistors. Low-voltage n-tanks are also used to enclose Schottky diodes.The high and low-voltage tanks described herein differ in the dopant concentrations used to create them, and are thus implanted into the chip at different times. The high voltage tanks are formed with a lower dopant concentration to preserve a high pn junction diode breakdown, but are deeper. The low-voltage tanks are shallower, but have a higher concentration of dopant.

At step 107, a plurality of high-voltage p-tanks are created in the epitaxial layer. The high-voltage p-tanks are used as tanks for devices such as 18 volt EEPROM gating transistors, EEPROM cells, and drain extended n-channel transistors. Step 108 involves the fabrication of low-voltage p-tanks as, for example, enclosures for low-voltage n-channel field effect logic transistors, as drains for extended drain p-channel field effect transistors, and as tanks for floating gate avalanche injection electrically programmable read-only memory (FAMOS EPROM) cells.

At step 109, one or more DWELL regions are masked and implanted to form self-aligned (p-) and (n+) diffusions. The (p-) portion of the DWELL is preferably formed by implanting boron into the exposed portion of the epitaxial layer, and the (n+) portion of the DWELL is preferably formed byimplanting arsenic into the same exposed portion of the epitaxial layer. The mask used to define the DWELL regions is preferably aligned using the alignment marks formed in ALIGN step 100. The DWELL regions serve as backgate and source of lateral DMOS and vertical DMOS transistors, as the base and emitter of vertical NPN transistors, and as the anode and cathodeof zener diodes.

At step 110, all tank/well dopants are diffused to nearly final depths by ahigh temperature operation.

At step 111, deep (p+) implantations are performed, for example, to form deep back gates for lateral and vertical DMOS n-channel power transistors.This step can be omitted if deep (p+) implantations are not needed.

At step 112, moat or isolating oxide regions surrounding active device areas are defined with a mask. Channel stop implants that further isolate the devices from each other are carried out at step 113 in selected (p-) regions, with self-alignment to the active device areas. In step 114, the previously defined isolating regions have oxide grown on the face of the semiconductor epitaxial layer.

Step 115 concerns growing a first gate oxide and the subsequent formation of a first-level polycrystalline silicon (poly 1) conductor. For example, floating gate avalanche injection "metal" oxide semiconductor (FAMOS) EPROM cell, double-level poly EEPROM cells, and poly-poly capacitors use first level polycrystalline silicon conductors.

Next, at step 116, the gate oxide for the control gates of the high-voltageand high power transistors is grown and threshold voltage V_(t) adjust implants are performed for these transistors. The V_(t) adjust forms a (p-) region at the surface of the DMOS transistors, self-aligned to activeareas by the field oxide.

At step 118, similar low-voltage V_(t) adjust implants are performed through the high-voltage gate oxide layer. For the low voltage transistors, the relatively thick high voltage gate oxide is removed and athin gate oxide is thermally gown.

Step 120 concerns a portion of the EEPROM cell fabrication, and includes an(n+) Fowler-Nordheim tunnel diode implant and the formation of a thin tunnel oxide over the implant.

At step 122, a second-level polysilicon (POLY 2) layer is deposited, doped,patterned and etched to define gates for low- and high-voltage field effecttransistors, single-level poly EEPROM cells, and to partially or completelydefine the control gate for FAMOS n-channel EPROM cells, double-level poly EEPROM cells and poly-poly capacitors.

At step 124, a patterning and etching of certain double-level poly gate stacks occurs to complete the definition of the FAMOS n-channel EPROM cellcontrol gates and floating gates, and, in one embodiment, stack-etched EEPROM cells. EPROM (n+) cell implants can be done with the stack pattern.

In step 126, several source/drain implants and diffusions occur. The polysilicon transistor and memory cell gates have sidewall oxides formed adjacent their lateral edges and cap oxides on other exposed polysilicon surfaces. A lightly doped drain (LDD) (n) type implant is made into the surface source/drain regions of the n-channel field effect transistors immediately before a main (n+) source/drain implantation. The LDD and (n+)implantations are annealed, followed by (p+) source/drain patterning and implantation. The n-type source/drain implantation step is further used toform surface contact regions for back gates for p-channel transistors, and the (p+) source/drain implantation step is further used to form surface contact regions for back gates for n-channel transistors.

In step 128, the formation of the semiconductor portions of the fabricated devices is essentially complete and all that is left is the conductive interconnection of these devices with each other and to the outside world,with the exception of Schottky diode devices. An oxide is deposited in step128 and is patterned and etched to produce contact windows.

At step 130, platinum is deposited and PtSi formed at contacts and Schottkydiodes. Next, the first level of metal is deposited, patterned and etched. At step 132, a second-level of insulator is deposited over the first metaland vias are formed to the first metal. The second metal itself is deposited, patterned and etched at step 134. The sequence of depositing aninsulator, forming vias, and forming metal interconnect can be continued asillustrated by steps 135 and 137. A protective overcoat is added at 136, and various post-clean room processes are performed on the chip at step 138.

FIGS. 2-7 relate the inventive process to particular device types. Structures which have similar appearance in the drawings retain the same reference number. For example, the field oxide regions shown in each of the figures is labeled reference numeral 210. Structures formed in the same process step but having different appearances are given different reference numbers. For example, compare the gate 244 in FIG. 2k to the gate 352 in FIG. 3a. Table 1 provides a map by which process steps of FIG.1 are related to FIGS. 2-7. For example, process step 101 forms an (n+) buried layer. In FIGS. 2a-2k, this layer has reference number 154; it is formed in vertical double-diffused "metal" oxide semiconductor (VDMOS) transistor 147. In FIGS. 4a-4b an (n+) buried layer has reference number 402 while in FIGS. 6a-6b it has reference number 602.

                  TABLE 1                                                          ______________________________________                                                       Figure           Ref.   Process                                  Structure     Number   Device  Number Step                                     ______________________________________                                         (n+) Buried Layer                                                                            2a-2k    147     154    101                                                    4a-4b    400     402                                                           6a-6b    600     602                                             High voltage (n) Tank                                                                        2a-2k    141     168    103                                                             145     169                                                                    146     170                                                                    147     171                                                           3a-3b    382     384                                                           4a-4b    400     404                                                           5a-5b    500     502                                                           6a-6b    600     604                                                           7a-7b    700     706                                             Deep (n+)     2a-2k    147     171    104                                                    4a-4b    400     430                                                           6a-6b    600     606                                             Low Voltage (n) Tank                                                                         2b-2k    139     175    106                                                             144     176                                                                    148     177                                             High Voltage (p) Tank                                                                        2c-2k    142     182    107                                                             143     184                                                                    144     180                                             Low Voltage (p) Tank                                                                         2d-2k    140     190    108                                                             145     192                                                                    149     186                                             DWELL (prior to                                                                              2e-2k    146     194    109                                      diffusion)             147     196                                             (n+) Portion of                                                                              2f-2k    146     159    110                                      the DWELL              147     163                                                           3a-3b    382     368                                                           4a-4b    400     420                                                           5a-5b    500     512                                                           6a-6b    600     620                                                           7a-7b    700     716                                             (p-) Portion of                                                                              2f-2k    146     157    110                                      the DWELL              147     161                                                           3a-3b    382     354                                                           4a-4b    400     414                                                           5a-5b    500     510                                                           6a-6b    600     618                                                           7a-7b    700     710                                             Deep (p+)     2f-2k    146     200    111                                                             147     202                                                           3a-3b    382     356                                                           4a-4b    400     426                                                           7a-7b    700     712                                             Patterned Nitride Layer                                                                      2h       all     204    112                                      (moat definition)                                                              Patterned Photoresist                                                                        2h       all     206    113                                      (channel stop def.)                                                            Field Oxide   2i-2k    all     210    114                                                    3a-3b    382                                                                   4a-4b    400                                                                   5a-5b    500                                                                   6a-6b    600                                                                   7a-7b    700                                                     Gate Oxide    2i-2k    149     212    115                                      FAMOS gate    2i-2k    149     214    115                                      Nitride/Oxide Layer                                                                          2i-2k    149     216    115                                      HV Gate Oxide Layer                                                                          2i-2k    141     218    116                                                             142                                                                            143                                                                            144                                                                            145                                                                            146                                                                            147                                                                   7a-7b    700                                                     HV V.sub.t Adjust                                                                            2i-2k    141     168    116                                                             142     182                                                                    144     176,180                                                                145     169                                                                    146     157                                                                    147     161                                                           3a-3b    382     350                                                           7a-7b    700                                                     LV V.sub.t Adjust                                                                            2i-2k    139     175    118                                                             140     190                                                           4a-4b    400     412                                             Gate Oxide Layer                                                                             2i-2k    139     218    118                                                             140     218                                                           3a-3b    382     370                                                           4a-4b    400     428                                             (n-) Tunnel Region                                                                           2i-2k    143     224    120                                      Tunnel Oxide  2j-2k    143     226    120                                      Poly Gates    2j-2k    139     228    122                                                             140     230                                                                    141     232                                                                    142     234                                                                    143     236,238                                                                144     240                                                                    145     242                                                                    146     244                                                                    147     246                                                                    149     248                                                           3a-3b    382     352                                                           4a-4b    400     410                                                           7a-7b    700     718                                             EPROM Stack   2k       139     248    124                                      LDD-Source/Drains                                                                            2k       140     254,256                                                                               126                                      (n+/n-)                142     258,260                                                                143     262,264                                                                144     266,268                                                                146     272                                                                    147     276                                                                    149     280,282                                                       3a-3b    382     394,398                                                                        358,360                                                       4a-4b    400     406,408                                                                        416,418                                                       5a-5b    500     504,506                                                                        514,516                                                       6a-6b    600     608,610                                                                        614,616                                                       7a-7b    700     702,704                                                                        708,720                                         Source/Drains (p+)                                                                           2k       139     284,286                                                                               126                                                             141     288,290                                                                145     292,294                                                                146     296                                                                    147     298                                                           3a-3b    382     362                                                           4a-4b    400     422                                                           5a-5b    500     415                                                           6a-6b    600     612                                                           7a-7b    700     714                                             Sidewall insulator                                                                           2k       all     250    126                                                    3a-3b    382                                                                   4a-4b    400                                                                   7a-7b    700                                                     Cap insulator 2k       all     252    126                                                    3a-3b    382                                                                   4a-4b    400                                                                   7a-7b    700                                                     BPSG {borophos-                                                                              2k       148     300    128                                      phosilcate glass}(only                                                         illustrated in device 148                                                      but preferably used in                                                         all devices in all figures)                                                    Orifice (only illustrated                                                                    2k       148     302    128                                      in device 148 but                                                              preferably used in all                                                         devices in all figures)                                                        Contact (only illustrated                                                                    2k       148     306    128                                      in device 148 but                                                              preferably used in all                                                         devices in all figures)                                                        Insulator     2k       148     304    128                                      ______________________________________                                    

FIGS. 2a-2k illustrate successive steps for forming several device types ona semiconductor substrate. A low-voltage p-channel field effect transistor 139, a low-voltage logic n-channel field effect transistor 140 (devices 139 and 140 being designed for voltages at or below about fourteen volts),a p-channel isolation or gating field-effect transistor 141 for an EEPROM array, an n-channel isolation or gating field-effect transistor 142 for anEEPROM array, electrically erasable programmable read-only memory Fowler-Nordheim tunneling cell 143, a drain-extended n-channel field effect transistor 144, a drain-extended p-channel field effect transistor 145, a lateral double-diffused "metal" oxide semiconductor (LDMOS) n-channel field effect transistor 146, a vertical double-diffused "metal" oxide semiconductor (VDMOS) n-channel field effect transistor 147, a Schottky diode 148 and a floating gate avalanche metal oxide semiconductor(FAMOS) electrically programmable read-only memory (EPROM) cell 149 may allbe formed on the same integrated circuit chip. Respective device areas and the devices themselves are denoted by common numbers throughout these drawings. Devices 141-147 and 149 are designed to be subjected to voltagesand/or current densities much greater than the low-voltage logic transistors 139 and 140.

While the various devices formed during this integrated process flow are shown in FIGS. 2a-2k as being closely adjacent each other, this would not necessarily be the case in the finished semiconductor chip 22. The devicesare shown in close association with each other only for the reader's convenience. The reader should understand that certain of the devices can (and most likely will) be separated by wide areas on the actual semiconductor chip 22. However, viewing the various devices in close association gives an understanding of the simultaneous application of eachprocess step to each of the devices fabricated according to the process.

A more detailed view of LDMOS transistor 146 of FIGS. 2a-2k is shown in FIGS. 3a and 3b. FIG. 3a is a cross-section through the length of the transistor, while FIG. 3b shows a plan view of part of a stripe taken fromthe width direction of the transistor.

In the prior art, LDMOS transistors are built with DWELL implants done after poly gate etching. This gives a source structure which has the channel length (defined by boron vs. arsenic diffusion) and the gate overlap of the channel defined by diffusion, hence a self-aligned source. The LDMOS transistor fabricated in the sequence of FIGS. 2a-2k has the DWELL defined and diffused prior to poly gate deposition. This advantageously puts the high temperature processing prior to MOS isolationformation, allowing VLSI density CMOS logic transistors to be built concurrently with the LDMOS power transistor.

Like LDMOS structure 146 of FIGS. 2a-2k, the LDMOS structure 382 of FIGS. 3a-3b has a double-diffused source/backgate, giving a well controlled channel length. Gate overlap of the source depends on photoalignment, which is excellent for submicron class steppers. LDMOS transistor 382 has a (p-) region extending to the edge of the drain-side field oxide as a consequence of the blanket (unpatterned) (p-) implant used to adjust high voltage MOS threshold voltages. The (n) source/drain regions are double diffused (n+/n-). The sidewall oxide on the source side of the poly gate self aligns the heavily doped regions of the source to the gate. FIG. 3b depicts a section of a stripe of an LDMOS transistor. Not shown are contact openings to the source/drain metal. Metal 1 runs along the stripe,parallel to the gate poly. Metal 2 can run either parallel or perpendicularto the gate, depending on chip global routing to bond pads. The HV-V_(t) adjust does not appear in the plan view since it is unpatterned. Termination of the stripes is semicircular to increase breakdown voltages.

Vertical DMOS transistor 400 (similar to VDMOS structure 147 of FIGS. 2a-2k) is shown in detail in FIGS. 4a and 4b. The source side of the transistor is identical to that of LDMOS transistor 382 of FIG. 3a-3b. Thedrain side includes a deep (n+) region to connect to an (n+) buried layer. This structure gives lower resistance, R_(DS) (on), because of less drain resistance, and much better isolation from the substrate since the heavily doped deep (n+) regions and (n+) buried layer greatly reduce the substrate PNP action (p backgate/n HV-NTANK/p/p-substrate).

A lateral NPN transistor is shown in detail in FIGS. 5a and 5b. Region 508 forms the base contact, region 506 forms the collector contact, and region514 forms the emitter contact.

A vertical NPN transistor is shown in detail in FIGS. 6a and 6b. Region 612forms the base contact, region 608 forms the collector contact, and region 616 forms the emitter contact.

A lateral diffused MOS n-channel transistor is shown in detail in FIGS. 7a and 7b.

While the invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various other embodiments will be apparent to persons skilled in the art upon reference to this description. It is therefore contemplated that the appended claims will cover any such modifications ofthe embodiments as fall within the true scope and spirit of the invention. 

What is claimed is:
 1. A method of forming a circuit device in a semiconductor layer comprising the steps of:exposing a region of said semiconductor layer; implanting p-type dopants into each said exposed region of said semiconductor layer, said p-type dopants having a first diffusion rate; implanting n-type dopants into said exposed region of said semiconductor layer, said n-type dopants having a second diffusion rate, said second diffusion rate different than said first diffusion rate; and heating said exposed region such that a first doped region is formed within a second doped region wherein said first doped region is doped with dopants of a first conductivity type and said second doped region is doped with dopants of a second conductivity type, said first conductivity type being different than said second conductivity type; wherein said circuit device comprises a D-well region of a lateral DMOS device which includes a source and a backgate, wherein said first doped region comprises said source and said second doped region comprises said backgate.
 2. The method of claim 1 wherein said dopants of a first conductivity type comprise n-type dopants and said dopants of a second conductivity type comprise p-type dopants.
 3. The method of claim 2 wherein said n-type dopants comprise arsenic dopants and said p-type dopants comprise boron dopants.
 4. The method of claim 1 and further comprising the step of forming a gate structure adjacent to and partially overlapping said backgate.
 5. The method of claim 4 and further comprising the step of forming first and second source/drain regions in said semiconductor layer, said first and second source/drain regions formed adjacent opposite ends of said gate structure.
 6. A method of forming a circuit device in a semiconductor layer comprising the steps of:exposing a region of said semiconductor layer; implanting p-type dopants into each said exposed region of said semiconductor layer, said p-type dopants having a first diffusion rate; implanting n-type dopants into said exposed region of said semiconductor layer, said n-type dopants having a second diffusion rate, said second diffusion rate different than said first diffusion rate; and heating said exposed region such that a first doped region is formed within a second doped region wherein said first doped region is doped with dopants of a first conductivity type and said second doped region is doped with dopants of a second conductivity type, said first conductivity type being different than said second conductivity type; wherein said circuit device comprises a D-well region of a vertical DMOS device;said D-well region comprising said first and second doped regions.
 7. A method of forming a circuit device in a semiconductor layer comprising the steps of;exposing a region of said semiconductor layer; implanting p-type dopants into said exposed region of said semiconductor layer, said p-type dopants having a first diffusion rate; implanting n-type dopants into said exposed region of said semiconductor layer, said n-type dopants having a second diffusion rate, said second diffusion rate different than said first diffusion rate; and heating said exposed region such that a first doped region is formed within a second doped region wherein said first doped region is doped with dopants of a first conductivity type and said second doped region is doped with dopants of a second conductivity type, said first conductivity type being different than said second conductivity type; wherein said circuit device comprises a bipolar transistor, said bipolar transistor comprising said first and second doped regions.
 8. The method of claim 7 wherein said circuit device comprises an NPN transistor which includes an emitter and a base and wherein said first doped region comprises said emitter and said second doped region comprises said base.
 9. A method of forming a circuit device in a semiconductor layer comprising the steps of:exposing a region of said semiconductor layer; implanting p-type dopants into said exposed region of said semiconductor layer, said p-type dopants having a first diffusion rate; implanting n-type dopants into said exposed region of said semiconductor layer, said n-type dopants having a second diffusion rate, said second diffusion rate different than said first diffusion rate; and heating said exposed region such that a first doped region is formed within a second doped region wherein said first doped region is doped with dopants of a first conductivity type and said second doped region is doped with dopants of a second conductivity type, said first conductivity type being different than said second conductivity type; wherein said circuit device comprises a Zener diode which includes an anode and a cathode, said Zener diode comprising said first and second doped regions. 